Currently there are two prevalent basic technologies for image sensing with solid-state apparatus, such as in a television camera or document scanner: the charge-coupled device, or CCD, and CMOS. These two technologies have respective practical advantages and disadvantages. Recently, however, there has become available a new sensor technology which is intended to preserve the advantages of either CCDs or CMOS. This technology is known as "CMOS active pixel image sensors" or "depleted-gate photosensors," or most simply "photogates." In brief, a small single-stage CCD is fabricated for each photosensor, and the output of the single CCD stage is integrated with CMOS circuitry, such as a transfer circuit. The basic technology of constructing such photogates is disclosed in Mendis, Kemeny, and Fossum, "CMOS Active Pixel Image Sensor," IEEE Transactions on Electron Devices, Volume 41, No. 3, March 1994.
The basic structure of a photogate-based photosensor is as follows. There is disposed in a silicon structure one externally-biased area with an exposed surface, known as a photogate, which accepts light thereon. When the photogate is exposed to light, a charge is created in the depletion layer thereof. A transfer gate is disposed next to the photogate. When it is desired to transfer the charge from the photogate, a potential is applied to the transfer gate, thus deepening the potential well there. This deepening of the potential well in the transfer gate causes the charge in the photogate to spill into the transfer gate, according to the basic CCD method. This CCD-type charge transfer occurs only once in the process, and the charge spilled into the transfer gate is converted into a voltage with associated CMOS circuitry.
Although photogates have numerous advantages, such as small size, CMOS-compatibility and relative ease of fabrication, certain problems must still be addressed in order to incorporate this technology in, for example, a full-color document scanner. In one type of full-color document scanner, there are provided three separate linear arrays, each array incorporating a relatively large number of photosensors. Each separate linear array of photosensors is filtered with one primary color filter, such as red, blue, and green. The three primary-color-filter linear arrays are then exposed to an original document moving past, to record video signals based on the exposed document. Because each individual linear array is filtered with one primary color, the ultimate output is three color separations based on the original image.
One particular practical problem associated with photogates that has been observed is difficulty in absorbing short wavelengths of light, particularly blue light. Because short-wavelength light tends to be attenuated by a polysilicon overlayer which is common in photogates, it tends not to penetrate into the monocrystalline silicon where the electron-hole pairs are generated. A clear example of this deficiency in photogates is shown in European patent application EP-A2-757390. In this reference, a full-color cell is designed, with photogates used for receiving red and green light; however, the blue light is intended to be detected not by a photogate but by a separate photodiode. The fundamental design of this cell shows that there is a need in the prior art for a photogate design which is particularly useful in detecting short wavelengths of light.